Transformers and choke coils are, in general, electrotechnical inductive components which are used, in different technical fields, in electric or electronic circuits. Although transformers and chokes have a similar structure their fields of application differ. Chokes are low-impedance coils for reducing high-frequency currents on electric lines and are used in the field of the power supply of electric and electronic devices, in power electronics and high-frequency engineering. Transformers generally serve to increase or reduce alternating voltages. Usually, the input terminals and output terminals of transformers are galvanically separated.
The requirements to be fulfilled by electronic and electric circuits in modern applications frequently necessitate a miniaturization, based on the desire for more compact designs of electric and electronic components, lower losses and maximum capacities, along with a simultaneous flexible adjustment to different voltage sources. It is desirable in many applications, for example, that the operation of electric and electronic circuits is independent of fluctuations in a supply voltage. Moreover, an increasing miniaturization of electric and electronic circuits is possible only if it is ensured that losses and tolerances are kept as low as possible, or are largely compensated, during the production of individual components of electric and electronic circuits. As far as inductive components are concerned, e.g chokes and transformers, this means that properties predetermined for these components, e.g. geometric dimensions, and physical properties, e.g. inductance, heat conduction and the like, are subject to as few tolerances as possible, respectively, deviate from desired physical properties to a smallest possible extent. For the production of inductive components this means that tolerances in the production of magnetic cores are reduced and compensated.
In general, in the fabrication of inductive components, the production of magnetic cores is accompanied by production-induced tolerances which cannot be avoided despite all optimization. For example, if core bodies formed of ferrite material are sintered, length tolerances of +/−2.5% have to be expected as ferrite material experiences thermally induced changes of length in sintering processes. Therefore, if a magnetic core is to be formed of individual core bodies, which are made of a sintered ferrite material, it cannot be precluded that assembled magnetic cores are subject to tolerances in the range of +/−2.5% per core body, resulting in a tolerance of +/−5% for a magnetic core formed of two core bodies.
The tolerances lead above all to problems on the connection surfaces, such that not only the inductive properties are affected, but also mechanical properties are changed, e.g. the mechanical stability of the magnetic core, as will be explained below. The length tolerances occurring in core bodies lead to offset sections on the contact faces of the used core bodies during the production of magnetic cores, preventing a flush coupling of the contact faces. FIG. 1 schematically illustrates, in a cross-sectional view not true to scale, a formation of a magnetic core according to a double E-core configuration consisting of two core bodies 1 and 3. The core body 1 has, in this figure, two side legs 11, 15 and one center leg 13. Core body 3 correspondingly has two side legs 31, 35 and one center leg 33. A tolerance-induced deviation in the widths of the legs 11, 31 of core bodies 1 and 3 is schematically shown by reference number V1 of FIG. 1.
For the two core bodies 1 and 3 to be glued together in a defined and reproducible manner, despite deviation V1 as shown, both core bodies 1, 3 are abutted against a stop face 5 during the gluing process to carry out a core alignment. As shown in FIG. 1, the offset between the legs of core bodies 1 and 3 increases, as is shown by offset V2 with respect to the center legs 33 and 13 and by offset V3 with respect to the side legs 35 and 15. Despite the alignment at an outer core surface of both side legs 11 and 31 by means of the stop surface 5, the offset increases with an increasing distance from the stop surface 5 (in the direction of the normal towards the stop face 5), as is shown in FIG. 1. Thus, the magnetic core correspondingly formed of core bodies 1 and 3 shows a very strong asymmetry in its legs. It is to be noted that the magnetically active cross-sectional area decreases on the contact faces of the legs of both core bodies 1 and 3 along the magnetic core to one side of the magnetic core. This results in different values for the magnetic resistance in the core legs (11, 31), (13, 33) and (15, 35) and undesired sources for leakage fluxes in the magnetic core, so that the inductance for the magnetic core formed from core bodies 1 and 3 uncontrollably changes and, in particular, deviates from a desired inductance. The offset in the core legs, and the associated misalignment of the legs, respectively, legs not connected in a flush manner at the connection surfaces, also lead to structurally weak points at these sites, which cause poor mechanical properties, make the magnetic core more prone to damages, and can entail problems in processes that follow the production of the magnetic core. Consequently, it is no longer possible to ensure an exact setting of desired properties for the inductive component to be produced.
Based on the problems described above it is, therefore, desirable to provide a core body, a magnetic core formed of core bodies, and a method for producing a magnetic core in which tolerances are compensated.